Pressure Gauge Overpressure and Diaphragm Seal

[Morteza Sadeghinia]

Hi.
Overpressure protection of pressure gauges are 1.3 times of gauge range in general and overpressure protector (gauge saver) will be used for cases where process design pressure is higher than 1.3 times of gauge range.
But what if a diaphragm seal (either threaded or flanged) is required too? Can we use these 2 (gauge saver and diaphragm seal) together? If yes, what is the correct installation order? If not, what is the alternative solution?
Thanks and regards.

 

[danw2]

I can't recall ever having seen a gauge protector installed on a diaphragm seal. That does not mean it isn't done, just that I haven't seen it.

There could two issues that I can think of
1. Displacement volume
The seal would need to displace enough volume to actuate whatever the protector's mechanism is. Does the protector vendor know what the displacement volume of the protector is? Note the comments on displacement volume on Wika's seal guide (attached.

2. filling procedure.
The assembly would be vacuum evacuated and then back-filled with fill fluid. Any device using a check ball might present a challenge when backfilling the fill fluid. I'll call the guy I know who fills gauges tomorrow when he's back from vacation to see what he says.

As to assembly order, the gauge would mount on the gauge side (topside) of the protector and the seal would mount on the process side of the protector and assembly would be evacuated and filled like a normal gauge/seal assembly.

 

[Morteza Sadeghinia]

Thanks danw2 but I didn't find my answer. I'm asking if both diaphragm seal and overpressure protector (gauge saver) can be used together in services which gauge rangex1.3 is lower than medium design pressure. Is yes, what is the correct installation order and if not, what is the alternate solution?

 

[danw2]

Haven't been able to touch base with the seal guy. Probably next week.

But I do know that I've seen a couple instances where a DP transmitter was used instead of a gauge pressure transmitter because the DP is designed to operate at higher, over-range static pressures than the equivalent ranged gauge pressure transmitter. The DP's low side is just left open to atmosphere to get a gauge pressure reading.

You could put a diaphragm seal on the DP high side if you need isolation for corrosion or materials incompatibility purposes. the assembly would have an unbalanced environmental temperature shift since all temperature artifacts will be on only the high side, with no corresponding temperature shift on the low side.

 

[Morteza Sadeghinia]

My question was about pressure gauge and your reply is about pressure transmitter which is totally different.
Can anyone else respond, please.

 

[mk3223]

Are you trying to install a pressure gauge with both gauge saver and diaphragm seal?
If yes, just need to connect them with a nipple in between.
If not, you should contact the vendor and find a proper diaphragm seal whaich is suitable for the operating condition and won't cause overpressuring.

 

[Morteza Sadeghinia]

Dear mk3223,
Thanks for your prompt response. As I stated before, the design pressure of the process I'm asking about is way more than selected gauge range. If I choose wider range then resolution will not be correct. So I should have a gauge saver (pressure limit valve). Also I should use diaphragm seal as the fluid is corrosive and our Client insists to use diaphragm seal instead of having gauge with higher grade material.
So I need them both.
Your solution about having a nipple in between might work but there's another technical issue. The area between diaphragm seal and gauge itself should be filled with oil in case of using diaphragm seal. So any problem in this assembly leads to drain of the whole oil and re-fill. Is that correct? And one more thing...Is this the correct installation order? piping isolation valve --> drip (flushing) ring --> diaphragm seal --> nipple --> gauge saver (pressure limit valve) --> pressure gauge

 

[xnuke]

Assuming the gauge saver is able to tolerate the process fluid, I'd install: piping isolation valve --> drip (flushing) ring --> gauge saver (pressure limit valve) --> nipple --> diaphragm seal --> pressure gauge

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[mk3223]

The area between diaphragm seal and gauge itself should be filled with oil in case of using diaphragm seal.

The fluid/oil of the pressure gauge to diaphragm shall be part of the gauge assembly and follow the vendor's installation & Maintenance guidelines.

 

[danw2]

Finally talked to the seal guy. He has not done either snubbers (needle valve restriction) or the adjustable pressure setpoint, spring-loaded protectors, but sees no problem in putting either on the instrument side of a diaphragm seal, other than an adder fee for the added time for assembly and messing with the assembly to get the fill fluid liquid through all the way into the gauge.

 

[danw2]

It's not just "replace the 10 dollar gauge".

It's
- requisitioning a replacement gauge
- removing the diaphragm seal assembly from the process
- replacing the gauge
- vacuum the air/fill fluid out of the system
- backfill the gauge/seal assembly with full fluid
- test assembly, zero adjust if needed
- replace the assembly
- all the associated paperwork

 

[TiCl4]

Morteza Sadeghinia,

Pressure gauges with diaphragm seals can be built to withstand very high pressures; a quickly look through E&H offerings go up to ~15,000 psi MAWP.

Why not just get a pressure gauge that isn't a weak link in the system? Your gauge saver system is a mechanically actuated one. Are you going to replace it every 3-5 years or perform PMs on it? What will keep it from failing?

Not to be pedantic, but simply don't use a pressure gauge with an MAWP lower than the system design pressure. That's just asking for a containment failure down the line, gauge "saver" or no.

Surely if you can afford a gauge and saver, you can afford a gauge with the correct MAWP?

 

[Morteza Sadeghinia]

Dear TiCl4,
E&H doesn't have gauge in product line as far as I know.
For pressure transmitters you're absolutely right. But when it comes to pressure gauge, you're gonna have to use gauge saver because MAWP of a gauge is 1.3 of the range (for some special cases up to 2 times of gauge range).

 

[TiCl4]

Moreteza,

Ah, I misunderstood the gauge/transmitter distinction when recommending E&H. However, my original point still stands how of a pressure are you talking? A quick google search shows Ashcroft has certain diaphragm seal models that can withstand up to 10,000 psi. Surely you aren't running above that, are you?

https://www.ashcroft.com/products/pressure/diaphragm-seal-isolators/202-203-flange-diaphragm-seals/

 

[danw2]

Yes, the diaphragm seal can take the pressure, but the pressure transfers to the gauge's sensing element, which cannot take the pressure without distortion and damage.

The OP, Morteza Sadeghinia, is correct when he states that mechanical pressure gauges typically can only tolerate abou 1.3 times the max range of the gauge without damage.

 

[TiCl4]

Danw2,
It appears he wants to measure pressure in a certain range below normal design pressure, even though normal design pressure is higher than 1.3x the intended measurement range on the gauge. I would suggest this is an inherently poor idea, even with a gauge saver, due to the reliability concerns I mentioned above.

OP, the diaphragm -> gauge saver -> gauge should work, but you'll need to ensure sensing fluid completely fills the entirety of the assembly. If you are wanting a smaller range in order for accuracy, why not install a pressure transmitter and display (if you need local display)? That should have enough accuracy for whatever you need.

Maybe it' s just me, but installing a gauge with an MAWP lower than system design pressure seems like a poor idea to start with.